Some vehicle navigation devices have an integrated antenna, in which a GPS (Global Positioning System) antenna and an antenna for radio beacons of VICS (Vehicle Information and Communication System) are integrated. In the vehicle navigation device having such an integrated antenna, the integrated antenna and a circuit section are connected with each other by a cable. The circuit section is provided with a resistor through which an electric current flows into the integrated antenna to which power is being supplied. By detecting a drop voltage generated in the resistor because of the current flowing through a low noise amplifier (LNA), a connection state (open, normal or short) of the integrated antenna is detected.
However, with such a construction, in which the current flowing into the low noise amplifier of the integrated antenna is detected, depending on a circuit constant, the amount of current flowing through the resistor for detecting the current may be too small to detect the drop voltage of the resistor. In this case, to obtain a large drop voltage by the resistor for detecting the current, the resistance value must be set large.
In the case where the integrated antenna is provided with a transmitting/receiving element for the optical beacons, when an optical up-link signal is outputted to the integrated antenna, a large current flows thorough the transmitting/receiving element. As a result, a current larger than usual flows through the resistor for detecting the current. Therefore, when the resistance value of the resistor is large, the resistor is likely to be burned out by the large current flowing into the transmitting element for optical beacons when the up-link signal is transmitted. Moreover, the transmitting/receiving of optical beacons may be disabled because of noise.
In JP 2003-234705A, for example, the operation of optical beacons is diagnosed by switching circuits to send a transmission signal for diagnosis to a received signal processing circuit during diagnosis. In JP 2000-132793A, communication operation is confirmed by placing a reflector in front of the optical beacon and reflecting transmitting light of the optical beacon to input it to a receiving circuit. However, this does not solve the above drawback.